The invention relates to a device for the measurement of ultrasonic transit times in workpieces with an HF pulse generator which generates high-frequency wave trains with few oscillation periods. The HF pulse generator is connected with an ultrasonic transmitter which is in coupling contact for ultrasound transmission with the workpiece to be examined. An ultrasonic receiver is likewise in coupling contact for ultrasound reception with the workpiece to be examined. The output signal of the receiver acts on a zero-crossing comparator and on a gate circuit. A synchronization input of the gate circuit is connected with the HF pulse generator. The gate circuit is connected to the first input of an AND-member with the start/stop input of a counter. The second input of the AND-member is connected with the output signal of the zero-crossing comparator and the time gate of the gate circuit is positionable on predetermined zero crossings.
For the characterization of material joints and states there are used ultrasonic processes in which the elastic interaction between the ultrasonic wave passing through the workpiece and the material is utilized. The measurement is the transit time of the ultrasonic wave. By very precise detection of these transit times, with exact knowledge of the spatial formation of the workpiece, mechanical tension states can be determined in components, textures in rolling products or also the porosity of ceramics.
A prior art device of this type is known from the U.S. journal Ultrasonics, vol. 26, No. 5, pp. 256-258 (1988), in which the received ultra-sound echoes are resolved with a zero voltage comparator into a pulse sequence In the time space of the entry of the first ultrasonic echo., the second ultrasonic echo, or a further ultrasonic echo, in each case a time gate with adjustable gate width is positioned. From the large number of zero crossing impulses of the first ultrasound echo a zero crossing is selected and a counter is started. From the large number of zero crossing impulses of the second or of a further ultrasound echo another zero crossing is selected and the counter is stopped. This prior art device for the measurement of ultrasonic transmit times has the disadvantage that for a given measuring arrangement the gates must be manually adjusted
In case the ultrasonic transit times change to a relatively great degree, the selected zero crossing impulse leaves the preadjusted gate and the device interrupts the measurement value detection.
If the material to be examined or the testing head is changed, the parameters of the gates must be manually redetermined and stored. This time-intensive process must likewise be carried out on a change of the sample thickness of the material to be examined.
The U.S. Journal Rev.Sci.Instrum., Vol. 51, pp. 355-356 (1980) teaches a similar device for the detection of ultrasound transit times with a zero crossing detector, in which the gate circuit is to be positioned manually by the operator with the aid of an oscilloscope.
From U.S. Pat. No. 3,690,154 there is known a device for the thickness determination of materials, which, with the aid of a tunnel diode, selects the first zero crossing of an ultrasound echo and uses it for the Starting and stopping of a counter. Such a device has the disadvantage of a low measuring accuracy, since the signal-to-noise ratio at the first zero crossing of the echo signal is relatively poor.
U.S. Pat. No. 4,452,085 describes a counting arrangement for an ultrasound scanner. A zero voltage detector counts the incoming echo pulses of a predetermined time segment over the number of which there is found the spectral attenuation of the reflected ultrasound in the sound-treated medium.
Another device for detecting ultrasound transit times is described by Moro, Farina and Rossi in the journal NDT-International, p. 169, Aug. 1980. An ultrasound wave of a few wave trains is generated by an excitation of a piezoceramic transformer with the aid of an HF transmitter. The ultrasound wave entering the workpiece generates by reflection of the workpiece back sides a first echo, and in sequence, a second and further echoes, which are received by the ultrasound receiver and amplified in the amplifier engaged on the outlet side.
In the gate circuits of the known devices, with the aid of synchronization impulses of the HF transmitter there are generated time gates with which the successively entering ultrasound echoes are coincident and generate starting and stopping impulses for the connected high-frequency counter. The overstepping of a threshold value of the amplified ultrasound signals is used for the starting and stopping of the time counter.
With this measuring method amplitude fluctuations which can arise through change of the coupling of the ultrasound transmitter or ultrasound receiver onto the workpiece to be investigated, through joint changes or by sound-bundle divergences, can lead to considerable errors in the time measurement. In the state of the art, through the errors arising here, it is not possible to make transit time measurements accurate to nanoseconds between two ultrasound echoes in an industrial environment.